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STEPPER MOTORS
Name
Designation
Department
Subject code
: Mr.R.Anandaraj
: Associate . Professor
: Electrical and Electronics Engineering
:EC 6252
Year
Unit
: II ECE A & B
: IV
Stepper Motor Basics
stator
N
S
N
S
S
rotor
N
Stator: made out of coils of
Electromagnet
wire called “winding”
Current switch in winding
==>Magnetic force
Rotor: magnet rotates on
==>hold the rotor in a position
bearings inside the stator
• Direct control of rotor position (no sensing needed)
printers
computer drives
• May oscillate around a desired orientation (resonance at low speeds)
• Low resolution
Increased Resolution
S
torque
N
S
N
Half stepping
angle
Increased Resolution
S
N
S
N
Half stepping
More teeth on rotor or stator
Increased Resolution
S
N
S
N
Half stepping
More teeth on rotor or stator
How to Control?
4 Lead Wire Configuration
Step Table
Step Red Blue Yellow White
0
+
+
1
+
+
2
+
+
3
+
+
4
+
+
-
A+
Red
4 lead
motor
ABlue
Yellow
B+
White
B-
Clockwise Facing Mounting End
Each step, like the second hand of a clock => tick, tick
Increase the frequency of the steps => continuous motion
Motoring along...
• direct control of position
• precise positioning (The amount of
rotational movement per step depends
on the construction of the motor)
• Easy to Control
• under-damping leads to oscillation at low speeds
• torque is lower at high speeds than the primary alternative…
DC motors -- exposed !
Position Sensors
 Optical Encoders
 Relative position
 Absolute position
 Other Sensors
 Resolver
 Potentiometer
Optical Encoders
• Relative position
light sensor
light emitter
grating
decode
circuitry
- direction
- resolution
Optical Encoders
mask/diffuser
• Relative position
light sensor
decode
circuitry
light emitter
grating
A diffuser tends to
smooth these signals
Ideal
Real
Optical Encoders
• Relative position
light sensor
light emitter
grating
decode
circuitry
- direction
- resolution
Optical Encoders
• Relative position
light sensor
light emitter
- direction
- resolution
decode
circuitry
grating
A
A
A lags B
B
B
Optical Encoders
• Relative position
light sensor
- direction
- resolution
decode
circuitry
light emitter
grating
Phase lag between A and B is 90 degree
A
B
A leads B
Optical Encoders
• Detecting absolute position
wires ?
Other Sensors
• Resolver
= driving a
stepper motor
• Potentiometer
= varying
resistance
Control
Control: getting motors to do what you want them to
What you want to control
For DC
motors:
=
what you can control
speed
voltage
windings’
resistance
N
V
N
R
w
V
S
e
back
emf
S
e
is a voltage generated by the rotor
windings cutting the magnetic field
emf: electromagnetic force
Controlling speed with voltage
• The back emf depends only on the motor speed.
e = ke w
• The motor’s torque depends only on the current, I.
t = kt I
R
V
e
DC motor model
Controlling speed with voltage
• The back emf depends only on the motor speed.
e = ke w
• The motor’s torque depends only on the current, I.
t = kt I
Istall = V/R
current when motor
is stalled
speed = 0
torque = max
V = IR + e
How is V related to w ?
tR
V=
+ ke w
kt
R
V
• Consider this circuit’s V:
e
- or -
V
w=- R t+
ke
kt ke
DC motor model
Speed is proportional to voltage.
speed vs. torque at a fixed voltage
speed w
V
ke
no torque at max speed
max torque when stalled
torque t
ktV
R
speed vs. torque at a fixed voltage
speed w
V
ke
no torque at max speed
Linear mechanical power Pm = F  v
Rotational version of Pm = t  w
torque t
ktV
R
stall torque
speed vs. torque at a fixed voltage
speed w
V
ke
Linear mechanical power Pm = F  v
Rotational version of Pm = t  w
max speed
power output
speed vs.
torque
torque t
ktV
R
stall torque
speed vs. torque
speed w
V
ke
gasoline engine
max speed
power output
speed vs.
torque
torque t
ktV
R
stall torque
Back to control
Basic input / output relationship:
tR
V=
+ ke w
kt
We can control the
voltage applied V.
We want a particular
motor speed w .
How to change the voltage?
V is usually controlled via PWM -- “pulse width modulation”
PWM
 PWM -- “pulse width modulation
 Duty cycle:
 The ratio of the “On time” and the “Off time” in one cycle
 Determines the fractional amount of full power delivered to the
motor